1. Field of the Invention
This invention pertains to mixtures of polyvinyl chloride (PVC) and polycarbonates which contain at least 15 mole % of fluorinated bisphenol monomer units (F-PC) such as 2,2-bis-(4-hydroxyphenyl)hexafluoropropane (6F-bisphenol A), herein referred to as 6F-PC. Blends of F-PC and PVC result in thermodynamically miscible, single phase systems which are transparent throughout all compositions.
2. Discussion of the Background
Blends of polycarbonates and polyvinyl chlorides are known. Generally, polycarbonates exhibit properties such as high temperature stability, good dimensional stability, high impact strength, good stiffness and most notably good transparency. For these reasons, PC is used in a variety of applications including glass replacement, housings, medical devices and containers. Nevertheless, PC does have drawbacks such as poor scratch resistance, poor long-term U.V. resistance and stress birefringence which have to be dealt with, particularly in demanding optical applications. Furthermore, typical polycarbonates like bisphenol A PC do not meet many local and federal requirements for flame retardancy.
Polyvinyl chloride, on the other hand, is widely used in many molding and coating applications including film, sheet, pipe, wire and bottles. It suffers from poor processability and brittleness and thus is often plasticized for many applications. This, however, limits its upper use temperature.
Blends of PC and PVC would therefore be expected to eliminate the individual deficiencies of the respective components and result in a material having considerably improved mechanical and optical properties for a wide range of applications. As a matter of fact, pure mixtures of PC and PVC are disclosed in U.S. Pat. No. 4,239,861. These mixtures are reported to have better resistance to deformation at elevated temperatures but suffer from brittle behavior and lack of transparency at most composition ratios.
Unfortunately, blends of PC and typical polyvinyl chlorides are not thermodynamically miscible at most compositions and their mixtures result in opaque materials which are not acceptable for transparent, optical applications.
The term "thermodynamically miscible" blend is used in the art to define a polymer blend that mixes on the molecular level so as to form a single, homogeneous phase which exhibits only one glass transition. In contrast, the term "mechanically compatible" is taken to mean that mixing of the polymers is on a small scale but larger than the molecular level. Furthermore, "mechanically compatible" implies that the multiple phases exhibit good adhesion to one another so as to yield good mechanical properties. Although both thermodynamically miscible and mechanically compatible blends exhibit good mechanical properties, a thermodynamically miscible blend will generally be stronger and only thermodynamically miscible blends are transparent, owing to their single phase nature.
Molding compositions of PC and PVC which are partially transparent are disclosed in U.S. Pat. No. 4,105,711. These blends are endowed with high dimensional stability and high impact strength because the polycarbonate contains more than 50 mole of a tetramethylated bisphenol monomer and because it has also been previously compounded with a rubber impact modifier.
Similarly, PC and PVC mixtures which are opaque but have good ductility (low brittleness) have been disclosed by U.S. Pat. No. 3,882,192. This invention teaches that addition of an ABS graft copolymer to the mixture affords ductility and has the further advantage of being melt processable in the 170 to 220.degree. C. range, avoiding degradation of the PVC. Moreover, it has been said (Battelle Report on Polymer Blends & Alloys--1986, Vol. II, page 288-289 and Chemical Week, pages 72-76, 1983) that such blends have too narrow a processing temperature range to assure a hypothetical commercial success.
Accordingly, the task existed to produce a PC/PVC blend which is transparent owing to the single phase nature of the component mixture, which demonstrates improved ductility over the prior art, and which can be melt processed at a wide temperature range for commercial viability and lack of PVC decomposition. This invention provides a miscible mixture of a polycarbonate and a polyvinyl chloride over all compositions which overcomes the disadvantages described above.